1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device which stores information in accordance with a charge amount in a charge storage layer.
2. Description of the Related Art
A NAND type flash memory in which data is electrically written and erased is known as a nonvolatile semiconductor memory device. Memory cells constituting the NAND type flash memory have a stacked gate structure. In this stacked gate structure, a first insulating film, a charge storage layer, a second insulating film and a control gate electrode are stacked in order on a silicon (Si) substrate. The first insulating film is called a “tunnel insulating film”. The second insulating film is called an “intergate insulating film” in the case where the charge storage layer is a floating gate electrode made of, for example, polysilicon, or the second insulating film is called a “block insulating film” in the case where the charge storage layer is made of an insulator.
Silicon oxide or silicon oxynitride has heretofore been used for the first and second insulating films. However, due to the miniaturization of the memory cells, the use of an insulating film with a higher dielectric constant (high-k film) is considered (e.g., JP-A 2003-68897 (KOKAI)). The reason for this is that a high-k film permits an increased electric thickness while maintaining an actual thickness (physical thickness).
However, defects tend to be formed in the high-k film as compared with the silicon oxide film and the silicon oxynitride film. Therefore, in the memory cell using the high-k films for the first or second insulating film, a leakage current in a high field region containing an FN tunnel current as the main component can be reduced in writing and erasure, whereas data retention characteristics deteriorate due to a leakage current in a low field region. Moreover, a high-k film of, for example, alumina or hafnia is apt to be crystallized due to a heat treatment provided in a process of manufacturing a memory cell. If the high-k film is miniaturized together with the miniaturization of the memory cell, the amount of a leakage current in the low field region more noticeably varies according to the size of a crystal grain and the number of crystal grain boundaries, resulting in decreased reliability of the memory cell. Thus, the disadvantage of the use of a crystallized high-k material is that the leakage current characteristics in both the high field region and the low field region cannot be improved at the same time.